1. Field of the Invention
The present invention relates generally to improvements in optical systems. More particularly, the present invention relates to improvements in wavelength division multiplexing (WDM) of optical signals.
2. Description of Prior Art
The transmission capacity of optical communication systems is presently limited by the optical source modulation bandwidth. Although optical fiber has a very broad transmission bandwidth, on the order of 10 to 20 THz, the system data rates transmitted over the fiber are presently limited to about 2.5 Gbits/sec for single-channel communication using typical optical sources such as wavelength-tuned distributed feedback (DFB) lasers. Wavelength division multiplexing (WDM) generally increases optical system capacity by simultaneously transmitting data on several optical carrier signals at different wavelengths. With simultaneous data transmission on each channel, the total system capacity is increased by a factor equivalent to the number of different wavelength channels.
As used herein, the term "WDM system" will generally refer to a system capable of simultaneously transmitting data on several wavelength channels. Other optical systems may utilize a single optical source to transmit data over several different wavelength channels at different times. Since usually only a single channel signal is transmitted at a given time in these other systems, the overall system capacity is not increased relative to that of a single-channel system. Thus, although certain optical sources may be tuned over a broad bandwidth, such that a single source could be used to transmit on several wavelength channels, the modulation bandwidth of the source still limits the total transmitted data rate.
Prior art WDM systems, which simultaneously transmit data signals on several channels, therefore generally include a separate optical signal source for each channel. For example, an array of laser diode signal sources may be used in a WDM system, with each laser diode source individually modulated by a different data stream. The modulated optical carrier signal wavelengths provided by the laser diode array are typically spaced evenly apart within the bandwidth of the optical fiber. The individually-modulated channel signals may be combined in an optical coupler or combiner and then supplied to one end of an optical fiber transmission path. At the other end of the fiber, a separate optical receiver is generally used for each of the wavelength channels. Each receiver typically includes an input filter tuned to a particular channel signal carrier wavelength, and a photodetector for demodulating the carrier signal to recover the original data stream.
Despite the substantially higher fiber bandwidth utilization provided by WDM systems, a number of serious problems must be overcome if these systems are to become commercially viable. For example, each optical source typically requires active stabilization in order to prevent cross-talk or overlap between adjacent channel signals. Currently available systems are usually actively stabilized at both the transmitter and the receiver. Additional system hardware and processing may be required to independently stabilize each channel source.
Another problem with existing WDM systems is the effect of chromatic dispersion. In optical fiber, for example, dispersion causes optical channel signals at different wavelengths to propagate through the fiber at different speeds. As a result, the data streams modulated on the different carrier wavelengths undergo relative time shifts, and system synchronization is therefore difficult to maintain. Known dispersion compensation techniques include placing sections of fiber with an offsetting dispersion characteristic in the fiber transmission path. This static compensation may alleviate dispersion in systems in which each channel signal travels the same distance. However, presently available techniques cannot provide adequate dispersion compensation in, for example, a practical optical network where each of the channel signals may have travelled through a different length of fiber.
The number of possible channels in prior art WDM systems may be limited in certain practical implementations. For example, the complexity of individually-stabilized laser diode sources limits practical laser diode arrays to about 10 to 20 diodes. In addition, the most efficient currently available photonic integrated circuits can be formed with only about four laser sources on a single chip. Packaging and source complexity constraints therefore represent a significant problem in present multi-source WDM systems. The complexity of each source also substantially increases the overall optical system cost. Although a large number of sources may permit large numbers of channels in principle, these practical considerations presently limit the channel density of WDM systems to about 20 channels or less.
The above problems limit use of bandwidth efficient WDM techniques in many applications. For example, optical interconnections between electronic circuits can provide a number of advantages, including lower cost, better flexibility, elimination of ground loops, reduced cross-talk, lower dissipation and improved signal-to-noise performance. However, optical interconnections typically involve a large number of different data signals, each of which may need to be modulated on a different channel signal. The different channel signals may then be passed through an optical transmission medium to another electronic circuit, where they are individually detected and converted to electrical signals for further processing. A separate optical source is generally required for each high-speed data signal to be interconnected. Very large scale integrated (VLSI) circuits, which may have a hundred or more high-speed signal lines requiring interconnection, would therefore need a prohibitively large number of sources to make use of WDM interconnects.
Although time domain multiplexing (TDM) of several lower data rate signal lines into a single high-speed data line might reduce the total number of sources required for WDM interconnects, high-speed electronic multiplexers are usually needed to perform this function. In modern high data rate VLSI circuits, optical source bandwidth limitations will generally permit TDM of only a few data signal lines for each source. In addition, clock skew problems may result because dispersion causes each optical channel wavelength to travel at a different speed through an optical fiber. It is therefore difficult to obtain the considerable advantages of optical interconnects using presently available WDM techniques.
As is apparent from the above, a need exists for efficient WDM of a large number of simultaneously-transmitted optical signals using only a single optical source, thereby avoiding the substantial cost, complexity, stabilization and dispersion compensation problems of the prior art, and permitting use of high-density WDM in applications such as optical interconnects and optical communication networks.